How do I limit wattage to incandescent bulbs

Discussion in 'General Electronics Chat' started by Ishan Perusinghe, Jun 28, 2015.

  1. Ishan Perusinghe

    Thread Starter New Member

    Jun 24, 2015
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    Hi there

    I have an array of 28 parrell wires rates at 40w each ( Edison bulbs ) powered by 240vac supply . Connected to a 1000w rates wall dimmer ( pot / switch ) . Now the total power of all the bulbs is 1160w . How can I limit the max wattage the bulbs can pull from the source to 1000w or is the pot already going to limit the circuit to 1000w . I understand the bulbs will be dimmer because of this .
     
  2. #12

    Expert

    Nov 30, 2010
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    28 x 40 = 1120 watts
    A wall dimmer is not a pot. It's a voltage chopper. Take out 3 bulbs or risk melting the dimmer.
     
  3. Ishan Perusinghe

    Thread Starter New Member

    Jun 24, 2015
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    The beauty of the light feature comes from the number of bulbs . Is there no way to limit the power draw from the bulbs. If I can't I will reduce the number of bulbs for safety as you suggest .
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Replace the Triac in the dimmer to a higher current model with a heat sink or look for a power tool version that has the higher current device.
    Max.
     
  5. #12

    Expert

    Nov 30, 2010
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    Buy another dimmer and use each one for half the bulbs...or buy a dimmer rated for the job if you want it all controlled by one knob. Much cheaper than buying a 250 watt resistor to cut the power down to 1000 watts.

    That's assuming it's nearly impossible to buy 35 watt light bulbs.
     
  6. BReeves

    Member

    Nov 24, 2012
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    Put a diode between the dimmer and the bulbs.
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Or 25-watt bulbs

    Or, you can buy dimmable LED bulbs. The quality and color is getting better and better. Each LED bulb that puts out the same amount of light as a 40-w incandescent will be about 7 watts.
     
    cmartinez and #12 like this.
  8. Ishan Perusinghe

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    Jun 24, 2015
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  9. GopherT

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    Nov 23, 2012
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    If you are in 50Hz part of the world, you may notice a flicker when you use a diode.

    Also, it will be a pretty big diode.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    The dimmer wouldn't like half-wave mains - but you could split the bulbs into 2 equal groups and feed each group with diodes pointing different ways.
     
  11. Ishan Perusinghe

    Thread Starter New Member

    Jun 24, 2015
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    Yeah I just realised it will take out half the sine wave . So flickering is an issue and could reduce bulb life .
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @Ishan Perusinghe
    If you are doing the wiring yourself, you can wire the bulbs in pairs in series (14 sets of 2 bulbs in series). They will each operate at half of their designed voltage and much below their designed current. You can easily use the dimmer you have and no need for a diode.

    Finally, operating at the lower level will let them last forever.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    Some thoughts:

    1) You could use a variac or other transformer to reduce the voltage from 240 VAC to something a bit less. Light bulbs are highly nonlinear so it's hard to tell you exactly what you need to do, but assuming that the bulbs don't change resistance too much over this range, you need (Vnew/Vold)² to be equal to (1000W/1120W), so you need Vnew to be about 227 VAC. However, this assumes that the limit on the dimmer is the power delivered to the load, which it isn't -- the dimmer neither knows nor cares about that. What it cares about is the amount of current it is delivering. So this would probably still stress the dimmer above its rating and you would need to lower it further.

    2) I would think that you could also put a suitably sized power resistor in series with the dimmer to drop some voltage. The dimmer rating of 1000 W (assuming that this is for a 240 VAC circuit)would seem to indicate that the max current rating was 4.17 A. Your total bulb resistance (at temperature) would appear to be 51.4Ω, so (making the big assumption that this doesn't go down too much for a 10% drop in power) at 4.17A this would yield a voltage of 214 VAC (this is the voltage you actually need to shoot for in #1 above). So to drop 26V at 4.17A would require a resistance of about 6.25Ω and the ability to dissipate about 110 W. If you could find two 12 V bulbs rated at about 50W or 60W and put them in series that might do it.

    3) The excess power consumption should only be a concern at the brighter settings, so you could mechanically limit the dimmer from being turned up above a certain level.

    4) The dimmer can almost certainly handle some amount of current above what it is rated for, so you could just accept the risk (though I'm definitely not recommending that).

    5) You can split the circuit into two sets of bulbs and put each set on its own dimmer. In generally you don't want to run electronic components at their limits -- it reduces their life expectancy markedly -- so this has the benefit that you are operating each dimmer at a much more reasonable level. If you really wanted to only have one knob to twiddle, there are ways to mechanically connect them so that turning one turns the other.

    6) You can use an electronic dimmer circuit that takes a control input (a pot or even a computer program, depending on how fancy you want to get) and drives all the bulbs appropriately.

    7) Use a lower wattage bulb. I don't know what the next size down available is, but you would be looking for something like 35 W max and, to make your dimmer nice and happy, something around 25W would probably be good.
     
  14. Ishan Perusinghe

    Thread Starter New Member

    Jun 24, 2015
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    Ahh that is a good idea . I can defo live with that . That would half the wattage . Thanks brain trust . Will post my finished project to show u guys .
     
  15. ian field

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    Oct 27, 2012
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    You could give each group of bulbs that I suggested, its own reservoir electrolytic. That would stop the flicker - but may upset the phase control a bit, I think you'd probably have to size the electrolytics by trial & error so they're just enough to smooth out the flicker without other ill effects.

    You'd probably have to turn the dimmer further before the lights came up and the top of the range would be cramped, but it would have to be some compromise between that and flicker.
     
  16. Ishan Perusinghe

    Thread Starter New Member

    Jun 24, 2015
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    Thanks for the all input will need some time to digest it all . But gophert has the simplest solution I think . Wiring a pair of bulbs in series then adding them to the parrell circuit .
     
  17. ian field

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    Oct 27, 2012
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    If you're in a 110V area - just order in 230V bulbs and up the wattage specification to get the brightness you want.
     
  18. #12

    Expert

    Nov 30, 2010
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    @ian field This is twice today you gave bad advice because you didn't read the Thread.
     
  19. ian field

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    Oct 27, 2012
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    The OP was long ago - I simply forgot what had been stated.

    And I did suggest; "if".
     
  20. #12

    Expert

    Nov 30, 2010
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    I'm not saying I've never done that, but you seem to have a run going today. ;)
     
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